FAA regulations require that an airliner engine be able to survive the impact of a 4-pound bird. The rules don't require that an engine continue to run normally after a four-pound bird ingestion, just that it can be safely shut down without exploding or disintegrating.

And the regulations don't address the problem of simultaneous multiple strikes when a plane encounters a large flock—which is what apparently happened to U.S. Airways Flight 1549. Moreover, birds like the Canada goose, whose population has grown in recent years, typically weigh 7 to 8 pounds; large swans can weigh as much as 25 pounds.

An engine shutdown is not a major emergency in a modern jet airliner. If only one engine was disabled by the bird strike, the plane should have been able to safely return for a landing at LaGuardia. So the early evidence suggests that both engines of the Airbus 320 were rendered useless by the collision.

Although the first jet airliners, the 707 and DC-8, had four engines, recent trends in airliner design favor two-engine configurations, even for very large aircraft like the Boeing 777. The reason is simple economics: Two big engines are cheaper to buy and maintain than four smaller ones.

The downside, however, is a lack of redundancy in a major bird strike. The trend toward very high-bypass ratio fanjet engines with large-diameter fans makes them more vulnerable to bird strikes as well.